Cellulase preparation containing nonionic surfactant and method of treating fiber

ABSTRACT

It is directed to a silicon carbide-based porous body, wherein said body is a porous one which contains silicon carbide particles as an aggregate and metallic silicon, and has an oxygen-containing phase at the surfaces of silicon carbide particles and/or metallic silicon or in the vicinity of the surfaces thereof. The silicon carbide-based porous body contains refractory particles such as silicon carbide particles or the like and yet can be produced at a relatively low firing temperature at a low cost, has a high thermal conductivity, and is superior in oxidation resistance, acid resistance, chemical resistance to ash and particulates.

TECHNICAL FIELD

[0001] The present invention relates to a silicon carbide-based porousbody used in a filter for purification of automobile exhaust gas, acatalyst carrier, or the like, as well as to a process for production ofsuch a silicon carbide-based porous body.

BACKGROUND ART

[0002] Porous honeycomb structures are in wide use as a filter forcapturing and removing the particulate substance present in adust-containing fluid (e.g. exhaust gas emitted from diesel engine), oras a catalyst carrier for loading thereon a catalyst component capableof purifying the harmful substances present in an exhaust gas. It isknown that as a material constituting such a honeycomb structure, thereare used refractory particles such as silicon carbide (SiC) particlesand the like.

[0003] As a specific technique related thereto, there is disclosed, in,for example, JP-A-6-182228, a porous, silicon carbide-based catalystcarrier of honeycomb structure, obtained by using, as a startingmaterial, a silicon carbide powder having a given specific surface areaand a given impurity content, molding the material into a desired shape,drying the molded material, and firing the resulting material in atemperature range of 1,600 to 2,200° C.

[0004] Meanwhile, there are disclosed, in JP-A-61-26550, a process forproducing a vitrifying material-containing refractory product, whichcomprises adding a vitrifying material to an easily oxidizable materialor a refractory composition containing an easily oxidizable material,mixing, kneading and molding them together with a binder, andopen-firing the molded material in a furnace containing a non-oxidativeatmosphere; and, in JP-A-8-165171, a silicon carbide molded materialobtained by adding, to a silicon carbide powder, an organic binder andinorganic binders of clay mineral series, glass series and lithiumsilicate series and molding the resulting material.

[0005] Also, in JP-A-6-182228 is introduced a process for producing aconventional porous, silicon carbide-based sintered body, whichcomprises adding, to silicon carbide particles as an aggregate, a bindersuch as vitreous flux, clayey material or the like, molding them, andfiring the molded material at a temperature at which the binder melts.

[0006] Further, as to a high-temperature use ceramic filter produced bymolding refractory particles which consist of silica sand, a groundpottery, a metal oxide (e.g. Al₂O₃, TiO₂ or ZrO₂), silicon carbide,nitride, boride, other refractory material, or the like and which areadjusted to a given grain size, to a porous, bottomed cylindricalmaterial using a refractory binder such as water glass, frit, glaze orthe like, there are disclosed, in JP-B-61-13845 and JP-B-61-13846, thepreferred average particle diameter and particle size distribution ofrefractory particles, the preferred porosity, average pore diameter,pore volume and partition wall thickness of cylindrical material, etc.In the sintering (necking between particles) caused by therecrystallization of silicon carbide powder per se, shown inJP-A-6-182228, the silicon carbide component vaporizes from the surfacesof silicon carbide particles and the vaporized silicon carbide componentcondenses at the contact areas (necks) between silicon carbideparticles; as a result, the necks grow and the particles are bonded toeach other. There are problems, however, that this method brings a highcost since a very high firing temperature is required to be employed inorder to vaporize silicon carbide, and that the yield after firing isreduced since a material of high thermal expansion coefficient isrequired to be fired at a high temperature as well.

[0007] Further in JP-A-2000-218165 are disclosed a honeycomb filterobtained by forming a silica film for increased strength, on the innerwalls of the pores of a porous silicon carbide sintered body, and aprocess for production of such a honeycomb filter. This honeycomb filterhas a sufficient mechanical strength but has yet undissolved problems inthe production cost, thermal conductivity, etc.; therefore, the fluidthereof is desired.

[0008] Meanwhile, the technique of bonding a silicon carbide powder (asa raw material) with a vitreous material, shown in JP-A-61-26550 andJP-A-6-182228 uses a low firing temperature of 1,000 to 1,400° C.;however, when the sintered body produced by the technique is used, forexample, as a diesel particulate filter (DPF) for removing theparticulates contained in the exhaust gas emitted from a diesel engineand the particulates collected by and deposited on the filter are burntfor reactivation of the filter, there occurs local heat generationcaused by the low thermal conductivity of the filter, which has incurredthe destruction of the filter. Further, the filter shown inJP-B-61-13845 and JP-B-61-13846 is porous but is a bottomed cylindricalmaterial having a large partition wall thickness of 5 to 20 mm;therefore, there is a problem that the filter is not usable under thehigh space velocity (SV) condition like as a filter for purification ofautomobile exhaust gas.

[0009] In order to solve the above problems, the present inventorspropose, in Japanese Patent Application No. 2000-113513, a poroushoneycomb structure containing refractory particles, in particular,silicon carbide as an aggregate and metallic silicon, and a process forproduction thereof. In the patent application, a honeycomb structure isproposed which can be produced at a relatively low firing temperature ata low cost and which has a high thermal conductivity, a sufficientporosity and a high specific surface area.

[0010] The honeycomb structure, however, may have problems dependingupon the special environment in which it is used and the manner in whichit is treated. For example, it is known that silicon carbide, whenheated under a low oxygen partial pressure, gives rise to oxidativedecomposition according to the following formula (1), resulting inreduced strength and oxidation resistance. It is also known thatmetallic silicon, when heated under a low oxygen content atmosphere,vaporizes or, as shown in the following formula (2), generates a SiOvapor. It is further known that these Si and SiO of gaseous state causeviolent heat generation when they are oxidized or carbonized.

SiC+O₂ → SiO ↑+CO ↑  (1)

Si+{fraction (1/20)}₂ → SiO ↑  (2)

[0011] When a filter is burned for reactivation, oxygen is consumed;resultantly, the filter is exposed to a reducing atmosphere. Therefore,when a silicon carbide-based honeycomb filter having a structure bondedby metallic silicon is used as a DPF and then is reactivated, oxidationreactions under a low oxygen partial pressure, such as shown by theabove formulas (1) and (2) may take place; and there has been a fear of,for example, the destruction of the filter caused by sharp temperatureincrease due to the oxidation of, in particular, metallic silicon.

[0012] Metallic silicon further has a property of easily dissolving inan acid when having no oxide film thereon. As a result, when a sinteredbody containing metallic silicon as a constituent is used as a DPF, thesintered body is exposed to an acidic gas atmosphere generated by thecombustion of sulfur, etc. present in the fuel used; and there has beena fear, for example, the destruction of the filter caused by dissolutionof metallic silicon.

[0013] In view of such a situation, the present invention aims atproviding a silicon carbide-based porous body which contains refractoryparticles such as silicon carbide particles or the like and yet can beproduced at a relatively low firing temperature at a low cost and whichhas a high thermal conductivity and is improved in oxidation resistance,acid resistance, chemical resistance to ash and particulates, andthermal shock resistance; a honeycomb structure which can be suitablyused, for example, as a filter for purification of automobile exhaustgas by a treatment such as clogging of through-channels at its inlet oroutlet, or as a catalyst carrier, even under a high SV condition; and aprocess for producing such a honeycomb structure.

Disclosure of the Invention

[0014] According to the present invention, there is provided a siliconcarbide-based porous body, characterized in that said body is a porousone which contains silicon carbide particles as an aggregate andmetallic silicon, and has an oxygen-containing phase at the surfaces ofsilicon carbide particles and/or metallic silicon or in the vicinity ofthe surfaces thereof.

[0015] In the present invention, the oxygen content is preferably 0.03to 15% by weight; the oxygen-containing phase is preferably amorphousand/or crystalline SiO₂ or SiO; and the following relation is satisfied:

A/B≧1.3

[0016] when the strength is taken as A (MPa) and the Young's modulus istaken as B (GPa).

[0017] According to the present invention, there is also provided ahoneycomb structure constituted by any of the above siliconcarbide-based porous bodies.

[0018] Meanwhile, according to the present invention, there is provideda process for producing a silicon carbide-based porous body,characterized by adding metallic silicon and an organic binder to rawmaterial silicon carbide particles, mixing them, molding the mixture toa predetermined shape, calcinating the molded material in anoxygen-containing atmosphere to remove the organic binder in the moldedmaterial, and firing the calcinated material to obtain a siliconcarbide-based porous body wherein an oxygen-containing phase is formedat the surfaces of the silicon carbide particles and/or the metallicsilicon or in the vicinity of the surfaces thereof.

[0019] According to the present invention, there is also provided aprocess for producing a silicon carbide-based porous body, characterizedby adding metallic silicon and an organic binder to raw material siliconcarbide particles, mixing them, molding the mixture to a predeterminedshape, calcinating the molded material to remove the organic binder inthe molded material, firing the calcinated material, and subjecting thefired material to a heat treatment in an oxygen-containing atmosphere toobtain a silicon carbide-based porous body wherein an oxygen-containingphase is formed at the surfaces of the silicon carbide particles and/orthe metallic silicon or in the vicinity of the surfaces thereof.

[0020] In the above present invention, the heat treatment is carried outpreferably in a temperature range of 500 to 1,400° C.

[0021] According to the present invention, there is further provided aprocess for producing a silicon carbide-based porous body, characterizedby adding metallic silicon and an organic binder to raw material siliconcarbide particles, mixing them, molding the mixture to a predeterminedshape, calcinating the molded material to remove the organic binder inthe molded material, firing the calcinated material, then coating thesurfaces of the silicon carbide particles and the metallic silicon witha fluid containing silicon and oxygen, thereafter subjecting theresulting material to a heat treatment to obtain a silicon carbide-basedporous body wherein an oxygen-containing phase is formed at the surfacesof the silicon carbide particles and/or the metallic silicon or in thevicinity of the surfaces thereof. In the aforementioned presentinvention, the heat treatment is carried out preferably in a temperaturerange of 50 to 1,400° C.

[0022] In each of the above-mentioned present processes for producing asilicon carbide-based porous body, the firing is carried out preferablyin a temperature range of 1,410 to 1,600° C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a graph obtained by plotting the strength (MPa)/Young'smodulus (GPa) ratios of honeycomb structures against the oxygen contents(weight %) in the honeycomb structures.

[0024]FIG. 2 is a graph obtained by plotting the thermal conductivities(W·(mK)−1) of honeycomb structures against the oxygen contents (weight%) in the honeycomb structures.

BEST MODE FOR CARRYING OUT THE INVENTION

[0025] The mode for carrying out the present invention is describedbelow. However, it should be understood that the present invention isnot restricted to the following mode alone and can be appropriatelysubjected to design change, improvement, etc. based on the ordinaryknowledge of those skilled in the art, as long as there is no deviationfrom the scope of the present invention.

[0026] The silicon carbide-based porous body of the present inventioncontains silicon carbide particles as an aggregate and metallic silicon;therefore, it can be produced at a relatively low firing (sintering)temperature, at a low cost and at an improved yield. Further, thepresent silicon carbide-based porous body uses metallic silicon forbonding between silicon carbide particles which are refractoryparticles; therefore, it has a high thermal conductivity as comparedwith conventional structures using a vitreous material for bondingbetween refractory particles and accordingly, when used, for example, asa DPF and subjected to combustion of the particulates deposited on thefilter, for filter reactivation, there occurs no local temperatureincrease as to damage the filter.

[0027] Furthermore, the present silicon carbide-based porous bodycontains an oxygen-containing phase at the surface of the siliconcarbide particles and/or the metallic silicon or in the vicinity of thesurface; therefore, even when it is exposed to such a low-oxygen contentatmosphere and high temperatures as experienced when used as a DPF, theoxidative decomposition of the silicon carbide and the metallic siliconis suppressed. Thus, the silicon carbide-based porous body of thepresent invention has improved oxidation resistance; therefore, itundergoes, when used as a filter and subjected to reactivation, nodamage due to the heat generated, for example, by the oxidation of thesilicon carbide and the metallic silicon. Such an effect is obtained notonly when the oxygen-containing phase is present on the surface of thesilicon carbide particles and/or the metallic silicon but also when theoxygen-containing phase is present in the vicinity of the surface.

[0028] The oxygen content in the silicon carbide-based porous body ofthe present invention is preferably 0.03 to 15% by weight, morepreferably 0.5 to 10% by weight, particularly preferably 1.0 to 8.0% byweight. When the oxygen content is less than 0.03% by weight, there issubstantially no oxide film; as a result, the oxidation of the siliconcarbide or the metallic silicon takes place easily and there may appear,for example, the damage of filter caused by the dissolution of themetallic silicon in an acidic gas or the like; therefore, such an oxygencontent is not preferred. When the oxygen content is more than 15% byweight, the metallic silicon is oxidized and the bonded areas betweensilicon carbide particles become SiO₂, resulting in a reduction inthermal conductivity; therefore, such an oxygen content is notpreferred, either. Incidentally, when the honeycomb structure of thepresent invention is used as a DPF, its thermal conductivity issufficient at about 10 W/mK or more in order to avoid the localtemperature increase of the filter during the reactivation.

[0029] In the silicon carbide-based porous body of the presentinvention, the oxygen-containing phase is preferred to be amorphousand/or crystalline SiO₂ or SiO. Owing to the presence of a phase havingsuch a composition, on the surface of the silicon carbide particlesand/or the metallic silicon or in the vicinity of the surface, theporous body of the present invention is effectively improved inoxidation resistance, acid resistance, chemical resistance to ash andparticulates and thermal shock resistance.

[0030] The silicon carbide-based porous body of the present inventionpreferably satisfies a relation of

[0031] A/B ≧1.3, more preferably a relation of A/B ≧1.4, furtherpreferably a relation of A/B ≧1.45 when the strength of the porous bodyis taken as A (MPa) and the Young's modulus thereof is taken as B (GPa).An A/B of less than 1.3 is not preferred because the resulting porousbody has a low thermal shock resistance. Therefore, the siliconcarbide-based porous body of the present invention whose A/B isspecified in the above range, is superior in thermal shock resistance.

[0032] Meanwhile, the honeycomb structure according to the presentinvention is characterized by being constituted by the above-mentionedsilicon carbide-based porous body of the present invention. Reflectingthe properties of the silicon carbide-based porous body which is aconstituent, the honeycomb structure possesses superior oxidationresistance, acid resistance, chemical resistance to ash and particulatesand thermal shock resistance. Further, the present honeycomb structureis not a bottomed cylindrical material of large wall thickness such asshown in JP-B-61-13845 and JP-B-61-13846 but a porous honeycombstructure and, therefore, can be used, for example, as a DPF forcollecting and removing the particulates emitted from a diesel engine oras a catalyst carrier, under a high SV condition.

[0033] Next, description is made on a process for producing a siliconcarbide-based porous body of the present invention. In producing thesilicon carbide-based porous body of the present invention, first,metallic silicon and an organic binder are added to silicon carbideparticles, and they are mixed to obtain a mixed powder. Or, when thesilicon carbide-based porous body is produced in the shape of ahoneycomb structure, metallic silicon and an organic binder are added tosilicon carbide particles and they are mixed and kneaded to obtain areadily formable puddle. The raw materials for the silicon carbideparticles and the metallic silicon contain a very small amount ofimpurities such as Fe, Al, Ca and the like in some cases; however, theymay be used per se or after purification by chemical treatment such aschemical washing or the like. When the honeycomb structure is used as afilter, various pore-formers may be added at the time of preparation ofthe readily formable puddle in order to obtain a higher porosity.

[0034] The above-obtained mixed powder or readily formable puddle ismolded into a predetermined shape such as honeycomb shape or the like;the molded material is calcinated in an oxygen-containing atmosphere toremove the organic binder in the molded material, for debindering; then,firing is conducted; thereby can be produced a silicon carbide-basedporous body of predetermined shape wherein an oxygen-containing phase isformed at the surface of the silicon carbide particles and/or themetallic silicon or in the vicinity of the surface.

[0035] Therefore, by conducting calcination in an oxygen-containingatmosphere as mentioned above, oxidation reactions proceed according to,for example, the following formulas (3) and (4), whereby an oxide filmof silica is formed.

SiC+20₂ →SiO₂+CO₂↑  (3)

Si+O₂→SiO₂  (4)

[0036] Next, other process for producing a silicon carbide-based porousbody according to the present invention is described. That is, theabove-mentioned mixed powder or readily formable puddle is molded into apredetermined shape such as honeycomb shape or the like; the moldedmaterial is calcinated to remove the organic binder in the moldedmaterial, for debindering; then, firing is conducted; further, a heattreatment is carried out in an oxygen-containing atmosphere; thereby canbe produced a silicon carbide-based porous body of predetermined shapewherein an oxygen-containing phase is formed at the surface of thesilicon carbide particles and/or the metallic silicon or in the vicinityof the surface.

[0037] Incidentally, in the above process for producing a siliconcarbide-based porous body according to the present invention, the heattreatment in an oxygen-containing atmosphere is carried out preferablyat 500 to 1,400° C., more preferably at 550 to 1,350° C., furtherpreferably at 600 to 1,300° C. When the heat treatment is carried outbelow 500° C., the formation of the oxygen-containing phase isinsufficient; when the heat treatment is carried out above 1400° C., theheat treatment temperature is close to the melting point of metallicsilicon and the fired material may be unable to hold the shape;therefore, such heat treatment temperatures are not preferred. Thus,according to the process for producing a silicon carbide-based porousbody according to the present invention wherein the heat treatmenttemperature is specified as above, an oxygen-containing phase can beeffectively formed on the surfaces of the silicon carbide particles andthe metallic silicon.

[0038] Next, still other process for producing a silicon carbide-basedporous body according to the present invention is described. That is,the above-mentioned mixed powder or readily formable puddle is moldedinto a predetermined shape such as honeycomb shape or the like; themolded material is calcinated to remove the organic binder in the moldedmaterial, for debindering; then, firing is conducted; further, a fluidcontaining silicon and oxygen is coated on the surfaces of the siliconcarbide particles and the metallic silicon both constituting the moldedmaterial. Thereafter, a heat treatment is conducted, whereby can beproduced a silicon carbide-based porous body of predetermined shapewherein an oxygen-containing phase is formed at the surfaces of thesilicon carbide particles and the metallic silicon or in the vicinity ofthe surfaces. Thus, an intended silicon carbide-based porous body canalso be produced by using a coating fluid containing silicon and oxygen,unlike the case for forming an oxygen-containing phase by oxidation.

[0039] As the fluid containing silicon and oxygen, there can be used afluid composed mainly of, for example, a silicon alkoxide, a silica sol,water glass or the like. These main components may be mixed asnecessary. The heat treatment after coating can be conducted at 50 to1,400° C. for 10 minutes to 4 weeks. The thickness of theoxygen-containing phase formed by coating can be controlledappropriately by adjusting the silicon concentration in the fluid. Thethickness of the phase can be made large by repeating the immersion inthe fluid and the subsequent drying. The thickness of the phase can becontrolled also by controlling the speed of taking out the to-be-coatedmaterial from the fluid.

[0040] In the above process for producing a silicon carbide-based porousbody according to the present invention, the heat treatment conductedafter the coating of the surfaces of the silicon carbide particles andthe metallic silicon after the firing is carried out preferably at 50 to1,400° C., more preferably at 100 to 1,300° C., further preferably at150 to 1,200° C. When the heat treatment is carried out below 50° C., along time is required before an oxygen-containing phase is formedsufficiently on the surfaces of the silicon carbide particles and themetallic silicon; when the heat treatment is carried out above 1,400°C., the heat treatment temperature is close to the melting point of themetallic silicon and the to-be-heated material may be unable to hold theshape; therefore, such heat treatment temperatures are not preferred.Thus, according to the process for producing a silicon carbide-basedporous body according to the present invention wherein the heattreatment temperature is specified as above, an oxygen-containing phasecan be effectively formed on the surfaces of the silicon carbideparticles and the metallic silicon.

[0041] In the process for producing a silicon carbide-based porous bodyaccording to the present invention, calcination is carried outpreferably at a temperature lower than the melting point of the metallicsilicon. Specifically, the calcination may be conducted by once keepingthe material to be calcinated, at a predetermined temperature of about150 to 700° C., or by using a small temperature elevation rate of 50° C./hr or less in a predetermined temperature range. When the calcinationis conducted by once keeping the to-be-calcinated material at apredetermined temperature, the predetermined temperature may be onetemperature level or may be a plurality of temperature levels dependingupon the kind and amount of the organic binder used; when theto-be-calcinated material is kept at a plurality of temperature levels,the times of keeping at these temperature levels may be the same ordifferent. When the calcination is conducted by using a smalltemperature elevation rate, the small temperature elevation rate may beused only in one temperature range or in a plurality of temperatureranges; when the small temperature elevate rate is used in a pluralityof temperature ranges, the temperature elevation rates in thesetemperature ranges may be the same or different.

[0042] In order to obtain a structure in which the refractory particlesare bonded by the metallic silicon, the metallic silicon must soften.Since the melting point of the metallic silicon is 1,410° C., the firingtemperature used in the firing is preferably 1,410° C. or more. Theoptimum firing temperature is determined from the fine structure andproperties required for the fired material. However, when the firingtemperature is higher than 1,600° C., the metallic silicon vaporizes,making difficult the bonding between the silicon carbide particles viathe metallic silicon; therefore, the firing temperature is appropriately1,410 to 1,600° C., preferably 1,420 to 1,580° C.

[0043] Incidentally, the production processes employingrecrystallization, shown in the above-mentioned JP-A-6-182228 andJP-A-2000-218165 enable bonding between silicon carbide particles andproduce a sintered body of high thermal conductivity; however, sincesintering is allowed to take place by vaporization and condensation, asmentioned previously, and silicon carbide is vaporized, a firingtemperature higher than that used in the present production process isneeded and firing at 1,800° C. or mores ordinarily at 2,000° C. or moreis necessary in order to obtain a silicon carbide sintered body which isusable practically. Meanwhile, the silicon carbide-based porous bodyaccording to the present invention is entirely different from theabove-mentioned conventional silicon carbide sintered bodies in that inthe present silicon carbide-based porous body, the silicon carbideparticles as a constituent are bonded with each other via the metallicsilicon also as a constituent. That is, since the metallic silicon actsas a binder, a relation of A/B≧1.3 is attained in the present siliconcarbide-based porous body when the strength of the porous body is takenas A (MPa) and the Young's modulus thereof is taken as B (GPa). That is,the relation of A/B>1.3 is not attained in the case of theabove-mentioned conventional silicon carbide sintered bodies; thus, inthe case of the present inventive silicon carbide-based porous body andthe process for production thereof, one may say that a consideration ismade also to the production cost, and that the present invention canprovide a silicon carbide-based porous body superior also in thermalshock resistance, and a honeycomb structure constituted by the porousbody.

[0044] The present invention is described in more detail below by way ofExamples. However, the present invention is in no way restricted tothese Examples.

(Example 1)

[0045] A SiC raw material powder having an average particle diameter of32.6 μm and a metallic Si powder having an average particle diameter of4 μm were compounded at a weight ratio of 80:20. To 100 parts by weightof the resulting powder were added 6 parts by weight of methyl celluloseas an organic binder, 2.5 parts by weight of a surfactant and 24 partsby weight of water. They were mixed and kneaded uniformly to obtain areadily formable puddle. The readily formable puddle was molded, usingan extruder, into a honeycomb shape having an outer diameter of 45 mm, alength of 120 mm, a partition wall thickness 0.43 mm and a cell densityof 100 cells/in.² (16 cells/cm²).

[0046] The honeycomb molded material was calcinated for debindering, ina low-oxygen content atmosphere at 550° C. for 3 hours and then fired ina non-oxidative atmosphere of reduced pressure at 1,450° C. for 2 hours,to obtain a silicon carbide sintered body of porous honeycomb structure.The crystal phase of the sintered body was examined by X-raydiffraction, which confirmed that the sintered body was composed of SiC,Si and a small amount of SiO₂.

(Example 2)

[0047] The operation up to molding was conducted in the same manner asin Example 1. The resulting honeycomb molded material was kept at 550°C. for 5 hours while air was allowed to flow; then, the air was switchedto Ar and the temperature was increased to 1,000° C. to conductcalcination for debindering, while keeping it as it was. Thereafter,firing was conducted in an Ar atmosphere at 1,450° C. for 2 hours toproduce a silicon carbide sintered body of porous honeycomb structure.The crystal phase of the sintered body was examined by X-raydiffraction, which confirmed that the sintered body was composed of SiC,Si and SiO₂.

(Examples 3 to 9)

[0048] The operation up to molding was conducted in the same manner asin Example 1. The resulting honeycomb molded material was calcinated fordebindering, in a low-oxygen content atmosphere at 550° C. for 3 hoursand then firing was conducted in a non-oxidative atmosphere of reducedpressure at 1,450° C. for 2 hours to produce silicon carbide sinteredbodies of porous honeycomb structure.

[0049] Each sintered body was heat-treated in air at 1,000 to 1,300° C.for 2 to 24 hours. There was no macroscopic or microscopic change in thehoneycomb structures before and after the heat treatment. The crystalphase of each sintered body after the heat treatment was examined byX-ray diffraction, which confirmed that the sintered body after the heattreatment was composed of SiC, Si and SiO₂.

(Examples 10 to 15)

[0050] The operation up to molding was conducted in the same manner asin Example 1. The resulting honeycomb molded material was calcinated fordebindering, in a low-oxygen content atmosphere at 550° C. for 3 hoursand then firing was conducted in a non-oxidative atmosphere at 1,450° C.for 2 hours to produce silicon carbide sintered bodies of poroushoneycomb structure.

[0051] Each sintered body was heat-treated in air at 1,000 to 1,300° C.for 2 to 24 hours. There was no macroscopic or microscopic change in thehoneycomb structures before and after the heat treatment. The crystalphase of each sintered body after the heat treatment was examined byX-ray diffraction, which confirmed that the sintered body after the heattreatment was composed of SiC, Si and SiO₂.

(Examples 16 and 17)

[0052] The operation up to firing was conducted in the same manner as inExamples 3 to 9, to produce silicon carbide sintered bodies of poroushoneycomb structure. Then, each sintered body was immersed in a silicasol (SiO₂ content: 4 or 20% by weight) and taken out slowly to coat thesintered body with the silica sol. Successively, each coated sinteredbody was heat-treated in air at 750° C. for 1 hour to producesilica-coated silicon carbide sintered bodies of porous honeycombstructure.

[0053] There was no macroscopic or microscopic change in the honeycombstructures before and after the silica coating. The crystal phase ofeach sintered body after the silica coating was examined by X-raydiffraction, which confirmed that the sintered body after the silicacoating was composed of SiC, Si and SiO₂.

(Comparative Example 1)

[0054] The operation up to molding was conducted in the same manner asin Example 1. The resulting honeycomb molded material was kept in annon-oxidative atmosphere of reduced pressure at 550° C. for 5 hours andthen fired, as it was, at 1,450° C. for 2 hours to produce a siliconcarbide sintered body of porous honeycomb structure. The crystal phaseof the sintered body was examined by X-ray diffraction, which confirmedthat the sintered body was composed of SiC and Si.

(Comparative Example 2)

[0055] The operation up to firing was conducted in the same manner as inExamples 10 to 15 to produce silicon carbide sintered bodies of poroushoneycomb structure. Then, each sintered body was heat-treated in air at1,410° C. for 4 hours. There was no macroscopic or microscopic change inthe honeycomb structures before and after the heat treatment. Thecrystal phase of the sintered body after the heat treatment was examinedby X-ray diffraction, which confirmed that the material was composed ofSiC and SiO₂.

(Test on Physical Characteristics)

[0056] Test pieces were cut out from each of the sintered bodiesproduced in the above Examples 1 to 17 and Comparative Examples 1 to 2and measured for oxygen content by infrared spectroscopy after meltingthem in inert gas. The test pieces were also measured for strength by athree-point bending test under room temperature using a material testerand for Young's modulus (determined from the relation of stress andstrain) by a static elastic modulus test method; then, astrength/Young's modulus ratio was calculated. Measurement andcalculation were also made for thermal conductivity by a laser flashmethod and for the amount dissolved in acid from the weight change oftest piece when the test piece was immersed in 10 wt. % sulfuric acid at80° C. for 50 hours. The results are shown in Table 1. In Table 1,“characteristic step of production process” refers to a step for formingan oxygen-containing phase in Examples 1 to 17 and Comparative Example 1and, in Comparative Example 1, refers to a calcination and firing step.

[0057] Further, each test piece was heated to 1,400° C. in a low-oxygencontent atmosphere, i.e., an atmosphere of high-purity He flow tovisually observe the vaporization and oxidation of Si. Metallic siliconturns white when oxidized; therefore, the vaporization and oxidation ofSi was rated as “yes” when the test piece turned white, because theoxidation of silicon was judged to have taken place and as “no” when thetest piece showed no discoloration, because there was judged to be nooxidation of silicon. The results are shown in Table 1. In FIG. 1 isshown a graph obtained by plotting the strength/Young's modulus ratiosof honeycomb structures against the oxygen contents in the honeycombstructures. In FIG. 2 is shown a graph obtained by plotting the thermalconductivities of honeycomb structures against the oxygen contents inthe honeycomb structures. TABLE 1 Strength (MPa)/ Young's AmountCharacteristic Oxygen modulus dissolved Thermal Vaporization step ofproduction content (GPa) in acid conductivity and oxidation process (wt.%) ratio (wt. %) (W/mK) of Si Example 1 Calcination in 0.03 1.3 0 17 Nolow-oxygen content atmosphere Example 2 Calcination in 1.4 1.5 0 14 Noair flow Example 3 Heat treatment 0.5 1.7 0 13 NO after firing: 1000° C.× 2 hr Example 4 Heat treatment 1.3 1.7 0 15 No after firing: 1000° C. ×6 hr Example 5 Heat treatment 1.6 1.6 0 14 No after firing: 1100° C. × 6hr Example 6 Heat treatment 2.8 1.5 0 15 No after firing: 1200° C. × 6hr Example 7 Heat treatment 3.4 1.5 0 19 No after firing: 1200° C. × 12hr Example 8 Heat treatment 5.5 1.5 0 12 No after firing: 1200° C. × 24hr Example 9 Heat treatment 4.5 1.7 0 15 No after firing: 1300° C. × 6hr Example 10 Heat treatment 3.0 1.7 0 11 No after firing: 1000° C. × 6hr Example 11 Heat treatment 3.3 1.8 0 13 No after firing: 1100° C. × 6hr Example 12 Heat treatment 4.2 1.6 0 11 No after firing: 1200° C. × 6hr Example 13 Heat treatment 5.8 1.9 0 11 No after firing: 1200° C. × 12hr Example 14 Heat treatment 9.4 1.8 0 10 No after firing: 1200° C. × 24hr Example 15 Heat treatment 5.6 1.8 0 11 No after firing: 1300° C. × 6hr Example 16 Composition of 1.0 1.6 0 14 No coating fluid: SiO₂ 20 wt.% Example 17 Composition of 0.54 1.5 0 16 No coating fluid: SiO₂ 4 wt. %Comparative Calcination and <0.01 *1 1.2 0.2 17 Yes Example 1 firingunder reduced pressure Comparative Heat treatment 19.0 1.8 0 3 NoExample 2 after firing: 1410° C. × 4 hr

[0058] As is clear from Table 1, there was an increase in oxidationresistance by forming an oxygen-containing phase at the surface ofsilicon carbide particles and/or metallic silicon or in the vicinity ofthe surface. As is clear from Table 1 and FIG. 1, there was also anincrease in strength/Young's modulus ratio. It can be confirmed fromTable 1 that as to acid resistance, there is no problem regardless ofthe oxygen content level in honeycomb structure.

[0059] Meanwhile, as is clear from Table 1 and FIG. 2, thermalconductivity tends to decrease with an increase in oxygen content inhoneycomb structure. When the oxygen content was below 0.03% by weight,there was substantially no oxide film; therefore, oxidation of siliconcarbide or metallic silicon took place and dissolution in acid solutionin a very small amount was confirmed. When the oxygen content inhoneycomb structure exceeded 15% by weight, reduction in thermalconductivity was confirmed. This is considered to be caused by thatmetallic silicon was oxidized and the bonded areas between siliconcarbide particles became SiO₂.

[0060] When attention is paid to the strength/Young's modulus ratiosshown in FIG. 1, strength/Young's modulus ratio exceeds 1.5 when theoxygen content in honeycomb structure is 0.5% by weight or more;therefore, it is more preferable that the oxygen content is 0.5% byweight or more. As shown in FIG. 2, when the oxygen content exceeds 15%by weight, there is a reduction in thermal conductivity; this isbelieved to be due to that metallic silicon is oxidized and the bondedareas between silicon carbide particles become SiO₂. Incidentally, whenthe thermal conductivity of a honeycomb structure is 10 W/mK or less andsuch a honeycomb structure is used as a DPF, a very large thermal stressis generated and may cause, for example, the breakage of the honeycombstructure. Therefore, it is further preferable that the oxygen contentin honeycomb structure is 10% by weight or less.

Industrial Applicability

[0061] As described above, the silicon carbide-based porous body andhoneycomb structure of the present invention contain refractoryparticles such as silicon carbide particles or the like and yet can beproduced at a relatively low firing (sintering) temperature and,therefore, at a low production cost and at an increased yield; and canbe provided inexpensively. Having an oxygen-containing phase at thesurface of the silicon carbide particles and/or metallic silicon or inthe vicinity of the surface, the silicon carbide-based porous body andhoneycomb structure of the present invention have high thermalconductivity and are improved in oxidation resistance, acid resistance,chemical resistance to particulate and ash, thermal shock resistance,etc.; therefore, when used, for example, as a DPF and subjected tocombustion of the particulates deposited thereon, for filterreactivation, there is no localized heat generation such as to damagethe filter. Further, being porous, the present honeycomb structure canbe suitably used as a filter for purification of automobile exhaust gas,a catalyst carrier, etc., even under a high SV condition.

[0062] Further, according to the present process for producing a siliconcarbide-based porous body, an oxygen-containing phase can be formedpreferably on the surface of the silicon carbide particles and/or themetallic silicon or in the vicinity of the surface according to apredetermined step and condition.

1. A silicon carbide-based porous body, characterized in that said bodyis a porous one which contains silicon carbide particles as an aggregateand metallic silicon, and has an oxygen-containing phase at the surfacesof silicon carbide particles and/or metallic silicon or in the vicinityof the surfaces thereof.
 2. A silicon carbide-based porous bodyaccording to claim 1, wherein an oxygen content therein is 0.03 to 15%by weight.
 3. A silicon carbide-based porous body according to claim 1or 2, wherein the oxygen-containing phase is amorphous and/orcrystalline SiO₂ or SiO.
 4. A silicon carbide-based porous bodyaccording to any of claims 1 to 3, which satisfies the followingrelation: A/b ≧1.3 when the strength is taken as a (mpa) and the young'smodulus is taken as b (gpa):
 5. A honeycomb structure, characterized inthat said structure is constituted by a silicon carbide-based porousbody set forth in any of claims 1 to
 4. 6. A process for producing asilicon carbide-based porous body, characterized by adding metallicsilicon and an organic binder to raw material silicon carbide particles,mixing them, molding the mixture to a predetermined shape, calcinatingthe molded material in an oxygen-containing atmosphere to remove theorganic binder in the molded material, and firing the calcinatedmaterial to obtain a silicon carbide-based porous body wherein anoxygen-containing phase is formed at the surface of the silicon carbideparticles and/or the metallic silicon or in the vicinity of the surface.7. A process for producing a silicon carbide-based porous body,characterized by adding metallic silicon and an organic binder to rawmaterial silicon carbide particles, mixing them, molding the mixture toa predetermined shape, calcinating the molded material to remove theorganic binder in the molded material, firing the calcinated material,and subjecting the fired material to a heat treatment in anoxygen-containing atmosphere to obtain a silicon carbide-based porousbody wherein an oxygen-containing phase is formed at the surface of thesilicon carbide particles and/or the metallic silicon or in the vicinityof the surface.
 8. A process for producing a silicon carbide-basedporous body according to claim 7, wherein the heat treatment is carriedout in a temperature range of 500 to 1,400° C.
 9. A process forproducing a silicon carbide-based porous body, characterized by addingmetallic silicon and an organic binder to raw material silicon carbideparticles, mixing them, molding the mixture to a predetermined shape,calcinating the molded material to remove the organic binder in themolded material, firing the calcinated material, then coating thesurfaces of the silicon carbide particles and the metallic silicon witha fluid containing silicon and oxygen, thereafter subjecting theresulting material to a heat treatment to obtain a silicon carbide-basedporous body wherein an oxygen-containing phase is formed at the surfaceof the silicon carbide particles and/or the metallic silicon or in thevicinity of the surface.
 10. A process for producing a siliconcarbide-based porous body according to claim 9, wherein the heattreatment is carried out in a temperature range of 50 to 1,400° C.
 11. Aprocess for producing a silicon carbide-based porous body according toany of claims 6 to 10, wherein the firing is carried out in atemperature range of 1,410 to 1,600° C.